1. Field of the Invention
The present invention relates to a phase change memory device, and more particularly, to a programming method of controlling a write current applied to a phase change memory device according to the ambient temperature and a write driver circuit realizing the programming method.
2. Description of the Related Art
A Phase-change Random Access Memory (PRAM) is a non-volatile memory device that stores data using a Phase Change Material (PCM), such as Ge—Sb—Te (GST), whose phase and resistance change according to the ambient temperature.
Information may be stored in the PRAM when the GST changes into a crystalline phase or an amorphous phase according to the ambient temperature and/or the heating time. A high temperature, e.g., 900° C. or more, may applied to the PCM so as to change the phase of the PCM. The high temperature may be obtained from joule heating, generated from current flowing through a cell of the PRAM.
In a write operation, when a current is applied to the PCM to generate a temperature greater than or equal to its melting temperature and then the PCM is rapidly cooled down, the PCM becomes amorphous and data ‘1’ may be recorded in the PCM. In this case, the PCM is said to have entered a “reset” state.
If the PCM is heated at a temperature greater than or equal to a crystallization temperature, maintained for a given time, and cooled down, the PCM becomes crystalline and data ‘0’ may be recorded in the PCM. In this case, the PCM is said to have entered a “set” state.
In a read operation, a bit line and a word line may be selected to choose a particular memory cell. When an external current is applied to the PCM, a resistance of the PCM changes. When the resistance of the PCM changes, a voltage changes, thus allowing the expression of binary values ‘1’ and ‘0’.
In a PRAM cell, the write operation may be affected by the ambient temperature, since both the amount of a write current and a dynamic resistance of the PCM may change according to the ambient temperature.
In general, the higher the ambient temperature, the lower the performance of a transistor that drives the write current. As the transistor deteriorates, the amount of the write current and the dynamic resistance of the PCM also decrease.
Accordingly, thermal energy, i.e., temperature, which is generated from joule heating and is a direct cause of the phase change of the PCM cell, decreases, thus changing the GST into an unstable crystalline or amorphous state. Thus, the difference between a reset resistance and a set resistance becomes smaller and may cause errors during a read operation.
FIG. 1 illustrates a variation in a reset resistance R_RESET or a set resistance R_SET according to the ambient temperature. Referring to FIG. 1, a ratio of the reset resistance R_RESET to the set resistance R_SET decreases as the ambient temperature increases. The ratio may decrease because the PCM does not reach a crystallization temperature or an amorphization temperature as the ambient temperature becomes higher during the write operation.
Thus, as the ambient temperature becomes higher, a sensing margin between the reset state and the set state become smaller, thus causing possible malfunction of a PRAM.
FIG. 2A illustrates a variation in the write current according to the ambient temperature and FIG. 2B illustrates a variation in energy in the PCM according to the ambient temperature.
Referring to FIG. 2A, the higher the ambient temperature, the worse the performances of the driver transistors, namely, the smaller the write current applied to the PRAM cell.
In FIG. 2B, the energy denotes thermal energy that is generated from the joule heating. The thermal energy generated from the joule heating may be proportional to the square of a current flowing through the PRAM cell and a resistance of the PCM. The higher the ambient temperature, the smaller the current flowing through the PRAM cell, and the lower the resistance of the PCM. As shown, the thermal energy in the PRAM cell decreases as the ambient temperature increases.
When the PCM enters the reset state, the PCM is heated to a temperature greater than or equal to the melting temperature Tm and then changes into an amorphous phase. However, if the ambient temperature exceeds a given temperature, a reset current may not reach a desired reset current range and the temperature applied to the PRAM cell may be lower than the melting temperature, thus changing the PCM into an unstable amorphous state.
Likewise, if a set current does not fall within a desired set current range, the temperature applied to the PRAM cell may not reach a crystallization temperature Tc, thus changing the PCM into an unstable crystalline state.
As a result, when the temperature applied to the PCM is lower than the melting temperature Tm, the PCM does not completely change into the amorphous state. When the temperature applied to the PCM is lower than the crystallization temperature Tc, the PCM does not completely change into the crystalline state. In both cases, both the reset resistance and the set resistance are lowered during the read operation, which may cause an operation error, where the reset state may be erroneously detected as the set state.